AMSTERDAM, Oct 18 (Reuters) - At least 1,320 underage Dutch girls between the ages of 12 and 17 fall victim to sexual exploitation in the Netherlands each year, a report on human trafficking published on Wednesday showed.

That group makes up nearly half of female trafficking victims in the Dutch sex industry, Corinne Dettmeijer, National Rapporteur on Trafficking in Human Beings and Sexual Violence against Children, said in the study.

Dettmeijer said the report contained the first reliable statistics on human trafficking in the Netherlands and the first of their kind in Europe, but was concerned by a decline in cases indicating that fewer were being reported.

The 108-page report compiled both domestic and United Nations figures from 2012-2016.

The United Nations Office on Drugs and Crime, which cooperated on the report, is encouraging other countries to produce similar data, to help create a clearer international picture of the scale of the crimes.

The total number of human trafficking victims in the Netherlands is roughly five times higher than reported figures indicated, at about 6,250 cases per year, it said, "meaning that many victims stay out of sight of authorities and support agencies."

"The number is high, but what makes these statistics unique is that they show us what specific groups are falling prey to human trafficking," National Rapporteur Corinne Dettmeijer said in an interview. "It has exposed our blind spots."

In a report published in 2015, the European Commission said there had been over 30,000 victims of human trafficking between 2010-2012 across all EU member States, of which around 1,000 were child victims were trafficked for sexual exploitation.

Dettmeijer said it was known that a relatively high number of women from Central and Eastern Europe were being forced into the sex industry, but the number of underage Dutch girls was surprising.

The figures were also remarkable because the number of reported cases of trafficking has fallen sharply over the past five years, from nearly 1,300 in 2012 to below a thousand last year.

"I am very concerned about the falling number of reported cases," Dettmeijer said. "This means that an increasingly larger portion of trafficking of humans is going unreported."